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MOS Technology 6502 CPU

Daniel Knight - 2016.10.09

The personal computing revolution began with the Intel 8080 CPU. This 8-bit CPU was introduced in 1974 at 2 MHz and was the heart of the first kit computer, the MITS Altair 8800. But it was the far less costly 6502 CPU that drove the home computing market.

The Story Behind the 6502

Like the 8080 and Z-80, the 6502 is an 8-bit CPU capable of addressing 64 KB of system memory. And just as the Zilog Z-80 had been based on the earlier Intel 8080 CPU, the MOS Technology 6502 was based on the Motorola 6800 CPU (released the same year as the 8080, and both chips initially sold for $360). In fact, many of the engineers behind the 6502 had worked on the 6800 at Motorola.

The goal of the design team was “to put out a part that would replace and outperform the 6800, yet undersell it.” When the 6502 was introduced in the second half of 1975, it had a $25 price tag. In response, Motorola trimmed the price of the 6800 from $175 to $69 – still nearly three times the cost of the 6502. (MOS Technology also made a 6501 CPU that could be used to replace the 6800 on a logic board, but Motorola filed suit against the upstart company, which agreed to withdraw the 6501 and pay Motorola $200,000.)

In May 1976, Motorola again slashed the price of the 68000 CPU, this time to $35.

The Hobbyist Market

MOS Technology created two development systems that engineers could use to prototype computers and other devices. The MDT-650 was a single-board computer, as was the KIM-1, which became very popular among computer hobbyists. The KIM-1 soon had company when Steve Wozniak designed the Apple 1, Apple’s first computer and only kit computer.

The 6502-based Apple 1 launched Apple Computer, and the KIM-1 helped Commodore enter the personal computer market. Commodore’s Jack Tramiel was so impressed with the 6502 CPU that he bough MOS Technology to give Commodore a price advantage in the new market.

Commodore was launched by Tramiel in 1954 to sell portable typewriters. When Japanese portables took over that market in the late 1950s, Commodore switched to adding machines. In the 1960s, Japanese adding machines came to dominate that market, so Tramiel moved into electronic calculators. This time it was Texas Instruments that undermined Commodore, selling its own brand of calculators for less than Commodore and others paid TI for the chips alone. This is part of the reason Tramiel bought MOS Technology and also the reason that Commodore would later drive TI out of the home computer market. More on that in A History of Commodore’s 8-bit Computers and The 1983 Home Computer Price War.)

The Home Computer Market

Just as 1974 gave us the CPUs and 1976 gave us the hobbyist computers, 1977 gave us the first three lines of home computers. Apple launched the Apple II, a fully assembled computer with color graphics to replace the Apple 1 kit computer, in June 1977. Tandy Radio Shack gave us the TRS-80 in August 1977. And Commodore gave us the PET in October 1977.

Two of these, Apple and Commodore, used the 6502 CPU. They would be joined by Atari in 1979 and the BBC Micro in 1981, and the 650x design would also make its way into several early gaming consoles. Tandy Radio Shack would later introduce its Color Computer, built around the Motorola 6809 CPU, kind of a chip cousin of the 6502. The 8080, Z-80, and 8085 were more common in the business market, where CP/M would dominate.

The world’s best selling computer, the Commodore 64, was built around a slightly modified version of the 6502 known as the 6510. The 6510 allowed the C-64 to switch out its ROMs for full access to its entire 64 KB of system memory.

In addition, the Atari VCS and Nintendo Entertainment System were also built around members of the 650x CPU family.

Beyond 8-bit Computing

Western Design Center took the 8-bit 65C02, a CMOS version of the 6502, to the next level with its 65C816 CPU. The 65816 was fully backward compatible with the 65C02 but also included full 16-bit support along with 24-bit memory addressing, allowing it to go beyond the 64 KB limitation of the original 6502 and access up to 16 MB of system memory.

The 65816 is notable for its use in two popular devices. The Apple IIGS (late 1986) greatly improved upon the original Apple II line of computers with new graphic modes, the Esoniq SDP-1 32-channel digital sound chip, a 2.8 MHz CPU (vs. 1 MHz in most Apple II models), and memory ranging from 256 KB all the way up to 8 MB.

In fact, Apple made a deliberate choice to run the 14 MHz CPU in the IIGS at 2.8 GHz – it didn’t want the IIGS to be perceived as competition for the Macintosh, which was built around the Motorola 68000, a 32-bit CPU with a 16-bit memory bus running at 8 MHz.

The 65816 design also made its way into the late 1990 Super Nintendo Entertainment System (SNES), where it ran at 3.58 MHz. This was Nintendo’s fourth generation gaming console and was destined to be the best selling console of its era.

That’s All, Folks!

The 6502 architecture had a huge influence on the world of personal computing from 1976 until well into the 1990s, when Commodore went out of business, Atari left the 8-bit market, Apple discontinues its Apple II line, and gaming consoles moved to the fifth generation, eventually leaving the SNES behind.